This invention relates to a method for enhancing the sharpness of picture images used for color scanners for plate making, laser color printers, facsimile devices and so on.
In order to enhance the sharpness in picture images, it is a general practice to form a sharpness enhancing signal by multiplying the difference between a sharp signal and an unsharp signal obtained from the picture images with a constant which determines the degree of enhancement, and to add the sharpness enhancing signal to a picture image signal. When the picture images are in color, a picture image is separated through a three-color filter and three-color image signals thus obtained are processed by the sharpness enhancement respectively. If it is assumed that the input three-color picture image is expressed as Di (i=1, 2, 3), and a sharp signal is expressed as Si (i=1, 2, 3), and an unsharp signal is expressed as Ui (i=1, 2, 3), and the picture image signal expressed as Di' (i=1, 2, 3), then the sharpness enhancement process will be expressed as follows: EQU Di'=Di+Ki.multidot.(Si-Ui) (1)
wherein Ki denotes a constant parameter determining the degree of sharpness enhancement.
In the above mentioned method of the formula (1), however, it is necessary to form a sharpness enhancement signal Ki.multidot.(Si-Ui) for each of the three colors, thereby complicating a processing device. There has been proposed a method of representing the sharpness enhancement signal Ki.multidot.(Si-Ui) by any one of the above three colors in order to simplify the process expressed by the above formula (1). In the method, if the sharpness enhancement signal corresponding to the selected color is represented by Ki.multidot.(S.sub.0 -U.sub.0), then the picture image signal Di' after processing will become: EQU Di'=Di+Ki.multidot.(S.sub.0 -U.sub.0) (2)
However, such a simplifying method is detrimental as it causes trouble unless the constant parameter used to determine the degree of sharpness enhancement is suitably set. Examples will be shown below to facilitate understanding.
Let us think of a case where the picture image signals (D.sub.1, D.sub.2, D.sub.3) of three-colors have the waveforms shown in FIGS. 1A and 1C. If a picture image signal D.sub.1 is used as a sharp signal S.sub.0, the unsharp signal U.sub.0 will have the waveform shown in FIG. 1B, and the difference between the sharp signal S.sub.0 and the unsharp signal U.sub.0 will be as shown in FIG. 1D. If a constant parameter used to determine the degree of sharpness enhancement is set as a constant Ki for all of the colors, the waveforms after processing will be as shown in FIGS. 1E and 1F. This means that the picture image signals D.sub.2 and D.sub.3 are enhanced in sharpness excessively so as to make the intensity ratio of three-color picture image signals (D.sub.1 ', D.sub.2 ', D.sub.3 ') different from that of original picture image signals (D.sub.1, D.sub.2, D.sub.3) on the edges of the picture image (the portion CN in FIGS. 1A to 1F), the edges being the transitions between different intensities of the picture image. The intensity ratio among three-color picture image signals generally determines the hue of picture images and therefore, if such a ratio changes, the hue will inevitably change. Due to such changes, the color on the periphery of the picture image will change or become dull.
In order to avoid such inconveniences, it is necessary to set the constant parameter Ki for the colors other than D.sub.1 at a smaller value than that used for D.sub.1. On the other hand, the constant parameter Ki for the colors other than D.sub.1 should be larger than that used for D.sub.1 in order to avoid a color change or dull color on the periphery of the picture image (the region CN in FIGS. 2A to 2F) when the waveforms of three-color picture image signals (D.sub.1, D.sub.2, D.sub.3) are as shown in FIGS. 2A and 2C. In order to prevent color changes or dullness in the conventional simplified method for the sharpness enhancement, the constant parameter Ki should be set in accordance with the intensity ratio of the three-color picture image signals. Moreover, as the intensity ratio of a picture image signal changes depending on the position of the image, the constant parameter Ki should be varied in dependance upon the position.